ENH: add non-const field access without triggering update counter (#1081)

- can now things like ref(), boundaryFieldRef(), primitiveFieldRef()
  with an optional argument that avoids triggering any update events

  Instead of

      Field<Type>& iF = const_cast<Field<Type>&>(fld.primitiveField());

  can now write

      Field<Type>& iF = fld.primitiveFieldRef(false);

  or simply

      auto& iF = fld.primitiveFieldRef(false);